New photonic tools for medical imaging can be used to understand the nonlinear behavior of laser light in human blood for theranostic applications. When light enters biological fluids, it spreads rapidly, but some cell suspensions may cause non-linear reactions in laser beams for self-focusing and increase light penetration into biomedical applications as a quantifiable disease marker. In a recent study, now published in Light: Science and Applications Rekha Gautam and her colleagues at San Francisco State University and an international team of employees showed that a laser beam shining through red blood cell suspensions could become "self-captured". The process reduced light distribution to maintain the beam of laser light within the biological samples.
The observed non-linearity depended on osmotic conditions and age of the samples. The researchers suggest using the technique to diagnose cell anemia or malaria; diseases that affect the size and shape of the blood cell. Osmotic conditions play an important role in the properties of human red blood cells (RBCs) crucial during disease analysis. Numerous efforts over the last decade have focused on the study of the biomechanical properties of RBC's suspended in various osmotic solutions. In the present work, Gautam et al. established the self-catching and scatter-resistant, non-linear propagation of a laser beam through three different osmotic solutions / conditions. The results showed that the strength of the optical nonlinearity increased with osmotic pressure on the cells. Interestingly, the non-linear behavior of aged blood samples with lysed cells was different due to the presence of free hemoglobin. To explain the experimental observations, Gautam et al. used a theoretical model with an optical power-mediated non-local non-linearity. The current work of light autonomous through scattered soft biological material can introduce new photonic tools for non-invasive biomedical imaging and medical diagnosis.
Human RBCs are disc-shaped malleable cells possessing a spatially uniform refractive index, as they lack nuclei as opposed to most organelles, and exhibit characteristic deformability of passage through veins and microcapsillaries. The change of change can be effected by modifying the osmolarity of the surrounding liquid buffer to use RBCs as matching optical fluid microlenses. The optical properties of RBC are important for in vitro and in vivo disease diagnostics, where the refractive index of RBC is determined by hemoglobin (Hb) – the major part of the erythrocyte dry weight content. As a result, if cell volume decreased due to varying osmotic conditions, the refractive index increased. Pathophysiological conditions such as cellular cell anemia, malaria and sepsis are often closely related to the physical properties of RBCs, their shape and size. The fundamental features of various refractive indices and cell forms allow the RBC & # 39; to respond to changes in various osmotic environments, making them ideal candidates for studying scatter light. In the present work, Gautam et al. showed non-linear self-uptake of light over one centimeter distance of propagation by spreading RBC suspensions. As they increased the laser beam effect, they showed the beam dramatically self-focus within all three osmotic conditions – like optical spatial solitons (non-linear, self-captured wave packs). The optical forces that change with cell density and morphology can provide non-invasive tools to sort different cells according to a particular phase of a given disease.
The researchers got blood samples from anonymous donors for the trials. In the first set of experiments, they used a linear polarized continuous wave (CW) laser beam with a wavelength of 532 nm. They focused the light in a 3 cm long glass cuvette filled with RBC suspensions in various osmotic conditions as previously described. They monitored the linear and non-linear outputs of the sample using a CCD camera and current detector and measured the beam diameters using the Beamview program. The beam initially diffracted with a low power of 10 mW and experienced strong scattering then due to random distribution of non-spherical shaped RBCs.
Gautam et al. then measured normalized laser transmission (output / input power) as a function of the input beam power. In hypotonic solutions, they noted that the RBCs were in a "raised" state where the effective refractive index of the cells decreased as the water-to-Hb ratio increased. In contrast, the researchers in the hypertonic solution observed that the RBCs shrunk and their effective index increased due to reduced water-to-Hb ratio. In a third isotonic solution, the cells displayed a "normal" state where the RBCs showed intermediate behavior. When the experiments were performed using the same blood samples two weeks later, the researchers particularly saw different results, with the nonlinear focus being dramatically improved for the hypertonic solution. Optical force-induced self-control light in human red blood cell suspensions Simulations of the optical force induced non-linear beam dynamics in RBC-like suspensions. The a-c Beam size (FWHM) changes as a function of the gradient and scattering forces obtained through numerical simulations using an input power of 350 mW and neglect of random spreading effects, observing the change in beam size when either the gradient or scatter force is "off". d, f Side view of beam propagation and e, g corresponding output transverse intensity patterns after propagation through a RBC-like random scatter medium at low (d, e) and high (f, g) beam power. The beam's side views and output intensity patterns are normalized relative to their respective maximum input power. Credit: Light: Science and applications, doi: 10.1038 / s41377-019-0142-1.
In another set of experiments, the researchers used a home-built optical tweezers system to measure the optical gradient force at RBC's. Gautam et al. collected the forward scatter light from the captured cells with a capacitor lens and then focused on a position sensitive detector (PSD). They calculated the stiffness and gradient strength of the three separate solutions. To simplify the measurements, Gautam et al. treated hypotonic and hypertonic RBCs are like disc-shaped objects. They used a CCD camera to record cell motions from the three different solutions along with a microscope with two objectives, where the setup was powered by a 960 nm laser beam. The results illustrated the movement of cells against brownish movement under the action of optical forces based on the conditions of the cell (shape, size) and their beam collection capacity. Gautam et al. estimated the catching strength by the Langevin equation and stated that the force followed a tendency to hypertonic> isotonic> hypotonic conditions.
The researchers then developed a model for simulating nonlinear radiation resistance in soft tissue to understand the optics of optical force-mediated nonlinearity. The modeled time evolution of the particle concentration distribution using a diffusion adduct equation and considered the presence of a forward force to push the particles along the beam direction, along with the optical gradient force. Gautam et al. calculated the change in beam size for the various gradient and scatter force parameters to simulate the non-linear self-focusing effects under different buffer conditions. They recorded the altered size, volume and refractive indexes of RBC's under various osmotic conditions that were responsible for the varying sizes of optical forces that altered the optical nonlinearity. The simulated results were qualitatively consistent with the experimental observations.
In this way, Gautam et al. investigated non-linear radiation propagation in human RBCs is suspended in three different buffer solutions. They found that RBC & # 39; is displaying a strong self-focusing non-linearity that could be chemically controlled based on the buffer solution. They therefore propose to adjust the optical non-linearity via osmosis and increased osmotic pressure outside the cells in fresh blood samples. When the samples age, free hemoglobin from the lysed RBCs played an active role in the observed optical non-linearity and improved the non-linear response during hypotonic conditions.
Using direct video microscopy and optical tweezers, the researchers found that the radiation capture strength was highest for RBC's under the hypertonic conditions and weakest for hypotonic solutions. The researchers introduced a theoretical model to validate the observed experimental effects. The work will introduce a new perspective in the development of diagnostic tools, as the results are very promising for the development of laser treatment treatments for blood related diseases.
New types of laser beam & # 39; through opaque media
Rekha Gautam et al. Optical power-induced nonlinearity and self-conducting light in human red blood cell suspensions, Light: Science & Applications (2019). DOI: 10,1038 / s41377-019-0142-1
I. M. Vellekoop et al. Utilization of the disorder for perfect focus, Nature Photonics (2010). DOI: 10,1038 / nphoton.2010.3
Roarke Horstmeyer et al. Wizard-assisted wavefront-shaping methods for focusing light in biological tissue, Nature Photonics (2015). DOI: 10.1038 / nphoton.2015.140
Roadmap on structured light. Journal of Optics . iopscience.iop.org/article/10. … 978/19/1/013001 / meta